Managed C++ class. Release OK, Debug NOK. Why?

[MB]

I had a C++ dll that I used for C++ applications. I added a managed C++
class to this library and started to use the managed C++ class in my C#
application.

This works OK only if I build the C++ dll in release mode. If I try to
build the C++ dll in Debug mode, I am unable to add the dll as a
reference to my C# project. I get an error message that:

"This is not a valid assembly or COM component".

It's strange that this same technique works if I build a release
version.

Can anyone explain why this is happening? Could it be related to the
fact that I did not start out the C++ project as a ".Net class
library"?

Thanks,

Mitch